The invention relates to an actuator of the type described below.
Such an actuator or controller which takes the form of a working cylinder and particularly a pneumatic working cylinder (see, for example, company brochure Festo Pneumatic, 386.7/90522LD, May 1992) constitutes a structurally simple solution for producing back-and-forth movements. Pneumatic devices are generally of uncomplicated construction, and therefore easy to service, operationally safe, reliable (possible lack of airtightness rarely has effects which interfere with functioning) not subject to overloading (force limit is determined by pressure intensity), relatively insensitive, easily controllable (by throttles and pressure regulating valves) and economical due to low production and maintenance costs, despite the relatively high production costs for compressed air. They feature low operating weight and only few structural components. Nevertheless, until now the use of pneumatic actuators remained limited to moderate operating temperatures. In particular, until now sealing problems prevented application at very high or very low temperatures.
It is conventional in the prior art to make such actuators with cylinder walls and pistons of metal. The sealing between cylinder wall and piston is normally provided by a piston skirt or an O-ring (see the above-mentioned company brochure, pages 20 and 21). The actuators can be unidirectionally or bidirectionally functioning working cylinders in which the piston is moved back and forth inside the working cylinder by supplying the workspace defined by the cylinder wall, the piston and a cylinder lid with compressed air and then again decompressing it. To move the piston back into its initial position, the decompressing is generally not sufficient, rather the return movement is effectuated by a spring, or by supplying compressed air to the other side of the piston (two-way functioning actuator, see o+p xe2x80x9coil hydraulic and pneumaticxe2x80x9d 31 (1987) no. 9, 718 to 724, especially 722).
To save on compressed air and to obtain in the workspace the high pressure required for high control forces it is necessary to minimize the leakage flows between the cylinder wall and the piston which is displaceable along the longitudinal axis of the cylinder. Indeed, by using special polymeric sealing materials (e.g. Teflon, Kalrez) good sealing effects can be achieved, yet the use of such polymers is normally limited to temperatures from about 50 to +200xc2x0 C. One can also envision making the space between piston and cylinder wall very narrow and employing for both components an alloy with equal thermal expansion coefficients. However the relatively high thermal expansion coefficient of metals has the effect that, even with equal expansion coefficients of piston and cylinder, the gap between them becomes substantially wider at relatively elevated temperatures and the efficiency becomes correspondingly poorer. In addition at both very low and very high temperatures the components of which the actuator is constructed contract or expand unequally and thereby cause the piston and the cylinder, or the piston rod where it traverses the cylinder lid to jam so that the actuator becomes inoperative.
The subject of the invention is to so improve an actuator of the type described herein that trouble-free operation is assured even at very low and very high operating temperatures.
This object is accomplished in accordance with the invention by the characteristics set forth herein.
The invention, through the selection of material or materials with low thermal expansion for the cylinder housing, the piston and the head bearing between piston and piston rod makes it possible to provide a seal between piston and cylinder housing solely by means of a close fit without additional sealing elements. The low thermal coefficient of expansion of the materials employed insures that the leakage of an operating medium filling the working cylinder remains low even at very high temperatures. Conversely, at very low temperatures, seizing-up of the piston in the cylinder housing is prevented because the gap between cylinder housing and piston diminishes only slightly due to the temperature reduction. If the piston and the cylinder housing are made of ceramic material, the high hardness of this material assures that the gap between piston and cylinder housing does not widen even after prolonged operation. The head bearing which connects the piston rod and the piston and is moveable transversely to the piston""s longitudinal axis assures that no bending forces can arise between piston rod and piston even at the required close tolerances.
If, in one embodiment of the invention, the piston rod of the actuator consists of material or materials with low thermal expansion, especially ceramics, high operating reliability of this component is provided, even at extremely low and high temperatures.
If, in a further embodiment of the invention, the head bearing consists of a round disc in which a ball joint cavity has been created, in which a ball-shaped end of the piston rod is engaged, then the ball-shaped end of the piston rod can be displaced transversely to the piston rod""s longitudinal axis, which simultaneously makes it possible to transmit a strong force in the pushing direction. From DE-AS (German Patent Publication) 14 75 578 it is already known to make the end of the piston rod in a ball shape.
If, in a further embodiment of the invention, the ball-shaped end of the piston rod engages the ball joint cavity by means of a snap fit, then the radially moveable head bearing can be made inexpensively. Because, when pressure is exerted on the piston rod, the ball-shaped end of the piston rod and the ball joint cavity can have a large contact surface, strong forces can be transmitted in the pushing direction. The smaller contact surface provided by the snap fit during exposure of the piston rod to a pulling force is of lesser importance, because strong pulling forces do not occur during normal operation. Only when the actuator takes the form of a two-way functioning actuator, in which the compressed air is also supplied to the side of the piston at which the piston rod is attached, then the head bearing must also be capable of transmitting relatively strong pulling forces.
If, in a further embodiment of the invention, the round disc of the head bearing is subdivided along a transverse surface passing through the center of the ball joint cavity, then the ball joint cavity and the ball-shaped end of the piston rod can be made so that, even if the piston rod is subjected to a pull, the contact surface between the ball-shaped end of the piston rod and the ball joint cavity is great enough to transmit strong forces.
If, in a further embodiment of the invention, the piston is subdivided and is joined together after application of the head bearing by means of an external shrink-on collar, the round disc can simply be embedded in the piston and the two piston halves can be centered easily and with accurate fit.
If, in a further embodiment of the invention, the cylinder housing is subdivided into two, or more than two cylinder housing segments, which bear against each other at abutting locations and, at the one or more abutments, there is provided an additional external shrink-on collar, then the cylinder housing can be assembled from segments which are simple and inexpensive to manufacture. Especially if the cylinder housing is to be made of ceramic and if the actuator is to have a relatively long stroke, construction of the cylinder housing from several housing segments becomes economical.
If, in a further embodiment of the invention, the cylinder housing consists of a tube which is closed at its respective ends by a first or second flange cover, and the first and second flange covers are connected in pretensioned manner by tie rods, then the tube can be easily manufactured from a material (often difficult to shape) which has low thermal expansion. The construction of the cylinder housing from several simple components reduces the manufacturing costs, especially when ceramic is used.
If, in a further embodiment of the invention, the additional external shrink-on collar has holes paralleling the longitudinal axis of the working cylinder through which the tie rods or connectors of the tie rods can extend, then these are passed through those holes and it is also easy to provide, for example, an electrically conducting or electrically insulating mechanical connection between the tie rods or the connectors and the additional external shrink-on collar.
If, in a further embodiment of the invention, the cylinder housing and the piston consists of ceramic such as SiSiC, the thermal expansion of both components is very low and the tolerance between the two components is then only slightly dependent on temperature.
If, in a further embodiment of the invention, the two flange covers are made of the same or of different ceramic such as SiN or SiSiC, the thermal expansion of the flange covers is low and has an order of magnitude similar to that of the thermal expansion of a ceramic tube. In this way, one can prevent relative motion of a head end of the cylinder housing relative to a flange cover because, first, the absolute thermal expansion of both components in the radial direction is small, and secondly is approximately equally great. This makes sealing between the cylinder housing and the flange covers easier.
If, in a further embodiment of the invention, the piston rod consists of ceramic such as SiN, then it can be subjected to high mechanical loads.
If, in a further embodiment of the invention, the head bearing consists of graphite or of ceramic such as SiSiC or of metal-impregnated carbon ceramic, it can be subjected to heavy mechanical loads, it is self-lubricating, and it exhibits low friction.
If, in a further embodiment of the invention, the piston rod is passed into the cylinder housing through a bearing sleeve of metal-impregnated carbon ceramic, then this passage is in the form of a low-friction slide bearing which simultaneously provides a high sealing effect.
If, in a further embodiment of the invention, the cylinder housing consists of electrically conductive or semiconductive material and is provided with electrical connecting terminals for connection to an electrical voltage source, then the cylinder housing can be used as an electrical resistance heater for heating the actuator.
If, in a further embodiment of the invention, an additional electrical connecting terminal is provided at the free end of the piston rod, then it is possible to electrically ascertain the location of the piston inside the cylinder housing by measuring the electrical resistance between the electrical connecting terminals on the cylinder housing and the piston rod at various piston locations and determining a corresponding characteristic curve. Thereafter, the prevailing piston location can be read out through resistance measurement.
An illustrative example of the invention is described in more detail in what follows with reference to the drawing. There is shown in